The present invention relates to flexible polymeric film packaging structures with improved oxygen, flavor/odor, grease/oil and moisture barrier characteristics. More specifically, the invention relates to a film structure which includes a polymeric substrate, an oxygen barrier containing cross-linked polyvinyl alcohol ("PVOH") and a moisture barrier including a polyvinylidene ("PVdC") coating.
Certain polymeric films employed for the packaging of foods inherently permit the transmission of oxygen and moisture from the outside of the film to the inside of a food package wrapped with the film. Oxygen and/or moisture permit rapid deterioration of foods packaged in polymeric films. Exclusion of oxygen and moisture from packaged foods retards product spoilage. In certain applications, such as condiments packaging, it is important that the packaging film prevents the flavor of condiments from spreading outside. In other applications, it is important to protect the packaged contents against acquisition of odors or even other flavors from the outside. Therefore, film structures which can provide a barrier to oxygen, moisture, flavor/odor, and grease/oil loss or gain are highly desirable food packaging materials.
Attempts have been made in the past to provide polymeric films which have high oxygen, flavor/odor, grease/oil and moisture barrier. Some polymeric films disclosed in the prior art include PVOH. Polyvinyl alcohols are polymers containing (--CH.sub.2 --CHOH--) groups. They can be applied from water solution and are in many respects attractive materials for use as barrier coatings for thermoplastic polymer substrates. When applied to thermoplastic polymer surfaces which have been treated by any of a number of means to render them receptive to coatings, polyvinyl alcohols adhere tenaciously, are flexible, clear and transparent, provide an oxygen impermeable barrier, and are resistant to permeation by greases, oils and a wide variety of common organic solvents, such as esters, ethers, ketones, hydrocarbons and chlorinated hydrocarbons.
The advantage of water solubility in the application of PVOH as a barrier coating becomes its chief drawback, however, when the barrier coating has been formed on a thermoplastic polymer substrate. The poor water resistance of PVOH restricts its use as a barrier coating to those few applications where nearly anhydrous conditions prevail. Moisture may act to cause staining or wear due to friction, an unpleasant feel to the touch and a dull appearance. It is also known that oxygen and flavor/odor, grease/oil permeability of PVOH increases in proportion to its moisture content.
A number of methods have been known for increasing the water resistance of PVOH, but none provides entirely satisfactory results. In spite of various heretofore practiced methods, such as cross-linking PVOH with potassium dichromate, formaldehyde or phenol-formaldehyde resins, the polyvinyl alcohol still tends to soften, swell and lose adhering strength when exposed to water or even moisture vapor. Hence, barrier coatings consisting only of polyvinyl alcohol cannot be used where a substantial degree of water-resistance or moisture-proofness is required.
Accordingly, there is still a need in the art of packaging films to provide a film structure which has excellent oxygen, flavor/odor, grease/oil and moisture barrier characteristics.
It is, therefore, an object of the present invention to provide new packaging film structures containing a film which has excellent oxygen barrier and flavor/odor, grease/oil characteristics present in a film combination including cross-linked PVOH coated with a material having high moisture barrier characteristics, such as polyvinylidene chloride ("PVdC").